1. Field of the Invention
The invention relates to the field of AC power switching generally, and more specifically, to AC switching devices minimizing or substantially eliminating switch contact erosion.
2. Description of the Prior Art
One of the most significant problems encountered in the application of electric power switches, is the erosion of the contact material due to arcing encountered at the time of switch closure and again, at opening. The problem is aggravated when the load is reactive in character. Virtually all of the commonly used AC machines fall into this category in that they are inductive loads to a greater or lesser extent. Moreover, such AC machinery accounts for the great majority of the AC switching applications where substantial power levels are involved. Accordingly, the specification and the invention it describes, deals exclusively with alternating current power switching in single or poly-phase circuits.
Aside from the well known approaches of the prior art involving immersion of AC power switches in inert fluids, and aside from refinement of contact materials themselves, and still further, aside from the development of switch arc extinguishing devices, contact anti-bounce contrivances, and the development and application of vacuum switches notwithstanding, there has been in more recent prior art an effort made to exploit the precise controllability of solid state electronic devices as AC switching elements. Such solid state electronic components, particularly the so-called silicon-controlled rectifier have been applied in this connection. The silicon-controlled rectifier is commercially available in units with large current handling capability and reasonable voltage characteristics. The characteristics of the aforementioned silicon-controlled rectifier units have been employed as the sole switching element in AC circuits, and also in combination with conventional mechanically operated switch contacts.
Examples of the former, i.e., arrangements in which the solid state device (thyristor, SCR, etc.) is the switching element substantially without mechanical contact cooperation in the power leads, are U.S. Pat. Nos. 3,881,118; 3,551,701; 3,753,044; 3,456,133; 3,581,114.
Examples of the prior art in connection with hybrid switches are found in U.S. Pat. Nos. 3,812,382; 3,790,863; 3,706,087 and 3,421,023.
For purposes of this description, a hybrid switch is one employing electro-mechanical contacts and also solid state switching devices in the power carrying circuits.
Various of these references also teach the use of the highly controllable solid state device such as the thyristor or SCR, to effect a switch closing or opening at a voltage or current null along the isntantaneous waveform of the alternating voltage or current waveforms.
One of the significant disadvantages in the "all electronic" solid state switching arrangements, is the potential for exposure of the solid state device to voltage and current surges in the circuit between power line and load.
Moreover, the electronically controlled current path provided through the SCR or other solid state device, is subject to random or spurious firing. To deal with that problem, an ordinary mechanical switch contact has frequently been inserted in series with the solid state device, thereby eliminating the hazard of spurious current initation.
In either prior art case aforementioned however, it will be noted that the solid state device must be capable of carrying the full load current (drawn by the AC machine for example) over an extended period of time. This involves large and expensive solid state devices with massive heat sinks.
The manner in which the present invention deals with the disadvantages of prior art devices for the basic purpose, will be understood as this description proceeds.